Thin film capacitor

ABSTRACT

Ferroelectric PbZr x  T 1-x  O 3  (PZT) thin films are deposited on Pt coated Si substrates by using RF magnetron sputtering. A method for obtaining desirable stoichiometric PZT, the desired ferroelectric perovskite phase, and better dielectric properties using a PZT target with Pb/(Zr+Ti) ratio of 1.2 and depositing at 350° C., followed by thermal treatment at 620° C. for 30 min is disclosed. The structural and electrical properties of the PZT layer were further improved by a method of fabricating a novel multi-layer structure which combined the PZT thin film with nanolayers of BaTiO 3 . The method and device of the present invention provided reduced leakage current density while maintaining high relative effective dielectric constants.

This application was filed as a provisional application, Ser. No.60/030,097, filed Oct. 30, 1996, abandoned hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to thin film capacitors havinghigh dielectric constants and more particularly to thin film dielectricarticles comprising a thin film layer of ferroelectric material incombination with a thin film layer of an amorphously configured lowleakage dielectric material.

2. Description of the Related Art

In forming dielectric articles such as semiconductor integrated circuitdevices, it is desirable to utilize capacitive elements that have highcapacitance in small dimensioned, planar structures to improve theelectrical performance and particularly to improve the response ofintegrated memory circuits. A typical capacitor comprises a pair ofelectrode layers having dielectric material therebetween. Voltage isapplied across the electrode layers and a charge is stored in thecapacitor with the amount of charge being storable in the capacitor,e.g. the capacitance, being proportional to the opposing areas of theelectrodes and the dielectric material.

Capacitance has also been found to be inversely proportional to thethickness of the dielectric material, thus thin film capacitors aregenerally seen as a preferable means to achieve high performance.Problems still exit however, in optimizing the performance of thin filmcapacitors, so there is a continuing need to improve electricalproperties, such as attaining higher dielectric constants and loweringcharge dissipation factors.

Further, it is often highly desirable in the fabrication of electricaldevices such as capacitors to minimize leakage current while maximizingcapacitance per unit area.

It has been found that leakage current can be controlled by utilizingSchottky barriers such as lead zirconium titanate (hereinafter PZT) inconjunction with Platinum (hereinafter Pt) electrodes. However,interdiffusion between the PZT layer and bottom electrodes of capacitorsof this type has been a problem in that deterioration of the electricalproperties of the capacitors and reduced crystallinity of the PZT filmscan result. A preferred embodiment of the present invention provides astructure combining PZT and one or more BaTiO₃ (hereinafter BTO) layers.According to a preferred embodiment of the present invention, one ormore BaTiO₃ layers may be employed to suppress the interdiffusionbetween Pt electrodes and the Pb element of the PZT layer. In addition,the combination of PZT and BTO according to a preferred method of thepresent invention has a synergistic effect and acts to increase thecrystallinity of the PZT films. Thus, the present invention provides acapacitor having a high dielectric constant and lower leakage currentthan previously obtainable.

European Patent 46,868 discloses fabrication of capacitor structuresusing dielectrics having high dielectric constants and discusses some ofthe problems associated therewith, particularly the tendency ofdielectric materials having a high dielectric constant to degraderapidly at higher temperatures and their forming a capacitor structurethat includes dual dielectric layers, comprising a first dielectriclayer of silicon nitride or aluminum oxide and a second layer selectedfrom a specific group of selected metal oxides and titanates. Such duallayered dielectric capacitors are said to have high capacitance(epsilon/t>0.04) and satisfactory Pb and dielectric loss.

U.S. Pat. No. 4,734,340 discloses an improved thin film capacitorwherein a particularly thin film dielectric layer, having highdielectric capacitance, is deposited by a sputtering technique andcomprises a mixture of tantalum and titanium oxides.

U.S. Pat. No. 4,803,591 discloses an improved capacitor comprisinglayers of dielectric ceramic compositions of high dielectric constant.The ceramic compositions are characterized as comprising magnesiumdioxide together with barium titanate, niobium pentoxide and zinc oxide.The capacitors formed from such ceramic compositions are said to havehigh dielectric constant with decreased temperature dependency over awide temperature range.

U.S. Pat. No. 4,873,610 discloses a dielectric article having a laminateof plural thin film dielectric material layers, comprising a combinationof dielectric material layers, that have different temperaturecharacteristics of permittivity. The patent specifies that opposinglaminates constitute different dielectric compositions for attainingadjacent layers having different temperature characteristics ofpermittivity. The reference does not disclose or infer that layersconstituting the same dielectric material can have different temperaturecharacteristics of permittivity.

U.S. Pat. No. 4,931,897 discloses a semiconductor element and method ofmanufacture wherein a lower electrode, having a polycrystalline siliconfilm thereon, is treated so that the silicon film comprises an amorphoussilicon surface. A thin film of dielectric material is thereafterdeposited on the amorphous silicon surface in such a manner that theamorphous surface does not recrystallize to a polycrystalline form. Thestated objective of the patent is to produce an interface, between thepolycrystalline silicon film serving as the lower electrode and thedielectric film, that is flat and uniform to prevent pinholes andelectric field concentration. The reference does not disclose theformation of a dielectric film having an amorphous and a polycrystallinelayer.

Thus, though the prior art is replete with proposed solutions formanufacture of optimized and dielectric articles, such solutions havenot sufficiently met the ever increasing demand of the emerging industryfor their various uses.

SUMMARY OF THE INVENTION

The invention comprises a chemically and electronically stable thin filmcapacitor, having a high dielectric constant and small current leakage,prepared by a method comprising forming a thin film layer of anamorphous dielectric material on the surface of a thin film layer offerroelectric dielectric material, the ferroelectric dielectric materialbeing a different dielectric material than the dielectric materialcomprising the amorphous film layer, and arranging the double layerbetween upper and lower electrodes. Favorable properties of PZT filmshave been achieved employing the teachings of the present invention. Forexample, PZT films produced in accordance with the principles of thepresent invention were observed to have leakage current 2×10⁻⁷ A/cm²@4×10⁵ V/cm, breakdown field (E_(B)) of 1×10⁶ V cm, dielectric constant(ε_(r)) of 548 and tanδ=0.03.

In a preferred embodiment, a thin film microcrystalline layer of asuitable dielectric material, preferably BaTiO₃ (BTO), is formed on asemiconductor substrate structure that includes a metal lower electrode.A polycrystalline layer of a suitable dielectric material, which ispreferably the same dielectric material as that comprising theabove-mentioned microcrystalline layer, and most preferably BTO, isformed on the microcrystalline layer. A layer of ferroelectric materialof high dielectric constant, the ferroelectric material being differentfrom any material comprising any other layer, the material preferablycomprising PbZr_(x) Ti_(1-x) O₃ (PZT), is formed on the polycrystallinelayer. Finally, an amorphous layer of a suitable dielectric material,preferably the same dielectric material as that comprising themicrocrystalline layer and preferably BTO, is formed on the layer offerroelectric material, the amorphous layer being in communication witha metal upper electrode.

An object of the instant invention is to provide dielectric articleswhich reduce the leakage problems associated with the use of the variousdielectric films.

Another object of the invention is to provide a thin film capacitor thathas improved resistance to leakage and has resistance to electric fieldconcentration.

A further object is to provide a method for the formation of thin filmdielectric articles that reduces leakage and/or electric fieldconcentration of the articles.

A still further object is to provide a thin film capacitor that hasimproved capacitance per unit film area.

A still further object of the invention is to provide a method for thepreparation of PZT films which results in a PZT film having improvedstoichiometric properties.

Still other objects and advantages of the present invention will becomereadily apparent to those skilled in this art from the followingdetailed description, wherein preferred embodiments are shown anddescribed, simply by way of illustration of the best mode contemplatedby the inventors for carrying out the invention. As will be realized,the invention is capable of other and different embodiments, and itsseveral details are capable of modifications in various obviousrespects, all without departing from the invention. Accordingly, thedrawings and description are to be regarded as illustrative in natureand not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the invention, as well as itscharacterizing features, reference should now be made to theaccompanying drawings wherein:

FIG. 1 shows the annealing temperature dependence of the Pb contentratio compared with the Pb/(Zr+Ti) with three different PZT targetcompositions.

FIG. 2 shows the XRD patterns for the PZT films deposited at (a) 25° C.,(b) 200° C., and (c) 350° C. without further thermal treatments.

FIG. 3 shows the XRD patterns for the PZT films deposited at differenttemperatures followed by different annealing temperatures (pyrochlorephase, pyr; and perovskite phase, pero).

FIG. 4a is a block diagram showing the general structural features andcomposition of a semiconductor capacitive element according to thepresent invention.

FIG. 4b is a block diagram showing the general structural features andcomposition of a first alternative embodiment of a semiconductorcapacitive element according to the present invention.

FIG. 4c is a block diagram showing the general structural features andcomposition of a preferred embodiment of a semiconductor capacitiveelement according to the present invention.

FIG. 5 is a sectional view of the preferred embodiment of asemiconductor capacitive element according to the present invention,including electrical contact means.

FIG. 6 shows the I-V characteristics of the single PZT and multi-layerstructures.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, FIGS. 4a, 4b and 4c are block diagramsshowing the general structural features of capacitors constructed inaccordance with the principles of the present invention.

In FIG. 4a, capacitor 200 comprises a metal lower electrode 219 engaginga semiconductor substrate 215 upon which metal lower electrode 219 isformed. In this embodiment capacitive element 200 comprises a substrate215, lower electrode means 219, upper electrode means 230 and two filmlayers, 225 and 227, of a dielectric material arranged in opposingjuxtaposition between upper electrode means 230 and lower electrodemeans 219.

Substrate 215 is preferably a (100)-oriented p-Si wafer which has beentreated or otherwise prepared to have deposited thereupon lowerelectrode means 219. Preferably, the surface of substrate 215 isprepared for deposition of the material comprising lower electrode means219 by cleaning and/or otherwise treating substrate 215 to removeimpurities, oxides and the like, and/or to create a smoothly refinedsurface to avoid pinholes from forming upon deposition of the electrodemeans material. Substrate 215 may be treated to provide a polishedsurface and oxidized in a wet O₂ atmosphere to provide an oxide layer(as best illustrated in FIG. 5 at 17). Other suitable substratestructures comprise materials such as SiC, GaAs, CdS, ZnO, ZnS or thelike.

Upper electrode means 230 and lower electrode means 219 generally may beconstructed from a suitably conductive metallic oxide or metal such asaluminum, copper, gold, silver, platinum, palladium, lead, ruthenium andmetallic oxides such as RuO₂ and the like. In a preferred embodimentupper and lower electrode means 230 and 219 comprise platinum (Pt). In apreferred embodiment the Pt layer comprising lower electrode means 219is a Pt film having a preferred thickness of about 200 nm. Inconstructing a capacitor in accordance with the principles of thepresent invention a (111)-oriented Pt film was utilized. However, thisorientation is not to be considered limiting, as other orientations maybe utilized with good results.

To construct the embodiment of FIG. 4a, the Pt layer comprising lowerelectrode means 219 may be deposited on the polished surface ofsubstrate 215 by means of RF magnetron sputtering carried out at asubstrate temperature of about 350° C. Other substrate temperaturesranging from about room temperature up to about 350° C. or higher may beused with acceptable results.

In the presently preferred embodiment, a pure Pt target of about oneinch diameter may be sputtered in an Argon (Ar) ambient atmospherehaving a base pressure greater than about 2×10⁻⁶ Torr. The total Arpressure and power during Pt deposition are preferably kept at about 10mTorr. and about 50 Watts respectively.

Dielectric film layer 225 is formed upon Pt layer 219. Dielectric filmlayer 225 is a ferroelectric material having a high dielectric constant,preferably a thin film layer of PbZr_(x) Ti_(1-x) O₃ (PZT) having apreferred thickness of about 200 nm. To arrive at the embodimentdepicted in FIG. 4a, PZT layer 225 is deposited upon Pt layer 219 bymeans of RF magnetron sputtering at a substrate temperature of about350° C. for about 30 minutes after which PZT layer 225 is preferablyfurnace annealed at about 620° C. for about 30 minutes.

Top amorphous layer 227 comprises an amorphous layer of a suitabledielectric material exhibiting low leakage current. Suitable dielectricmaterials which may be used in the invention are BaTiO₃, SrTiO₃, KNO₃,LiNbO₃, Bi₄ Ti₃ O₁₂, PbTiO₃, PbZrO₃, LaTiO₃, PbMgO₃, PbNbO₃, LaZrO₃, andthe like. A most preferred dielectric material for top amorphous layer227 is BaTiO₃ (BTO). Preferably, a thin film layer of BaTiO₃ (BTO)having a thickness of about 15 nm. is deposited upon PZT layer 225 bymeans of RF magnetron sputtering. Substrate temperatures duringsputtering can range from about room temperature up to about 350° C.

Upper metal electrode 230 may be formed upon top amorphous layer 227such that upper metal electrode 230 is in communication with topamorphous layer 227.

An alternative embodiment of a capacitor 100 constructed in accordancewith the principles of the present invention is depicted in FIG. 4b.Capacitor 100 comprises a metal lower electrode 119 engaging asemiconductor substrate 115 upon which metal lower electrode 119 isformed. In this embodiment capacitive element 100 comprises a substrate115, lower electrode means 119, upper electrode means 130 and three filmlayers, 127, 125 and 121, of a dielectric material, the film layersarranged in opposing juxtaposition between upper electrode means 130 andlower electrode means 119.

Substrate 115 of the preferred embodiment comprises a (100)-orientedp-Si wafer which has been treated or otherwise prepared to havedeposited thereupon lower electrode means 119. Preferably, the surfaceof substrate 115 is prepared for deposition of the material comprisinglower electrode means 119 by cleaning and/or otherwise treatingsubstrate 115 to remove impurities, oxides and the like, and/or tocreate a smoothly refined surface to avoid pinholes from forming upondeposition of the electrode means material. Substrate 115 may be treatedto provide a polished surface and may be, but is not necessarily,oxidized in a wet O₂ atmosphere. Other suitable substrate materialscomprise materials such as SiC, GaAs, CdS, ZnO, ZnS or the like.Substrate 15 may be a semiconductor substrate, or may be constructedfrom a dielectric, metal, or glass material.

Upper electrode means 130 and lower electrode means 119 generally may beconstructed from a suitably conductive metallic oxide or metal such asaluminum, copper, gold, silver, platinum, palladium, lead, ruthenium andmetallic oxides such as RuO₂ and the like. In a preferred embodimentupper and lower electrode means 130 and 119 comprise platinum (Pt). Inconstructing the capacitor of the preferred embodiment, the Pt layercomprising lower electrode means 119 was a (111)-oriented Pt film havinga preferred thickness of about 200 nm. However, other Pt orientationsand thicknesses may be employed without departing from the teachings ofthe present invention.

To construct the embodiment of FIG. 4b, the Pt layer comprising lowerelectrode means 119 may be deposited on the polished surface ofsubstrate 115 by means of RF magnetron sputtering carried out at asubstrate temperature which can be about room temperature up to about350° C. For example, a pure Pt target of about one inch diameter may besputtered in an Argon (Ar) ambient atmosphere having a base pressuregreater than about 2×10⁻⁶ Torr. The total Ar pressure and power duringPt deposition are preferably kept at about 10 mTorr. and about 50 Wattsrespectively.

Dielectric film layer 121 is formed upon Pt layer 119. Dielectric filmlayer 121 comprises a polycrystalline layer of suitable materialexhibiting low leakage current. Suitable materials exhibiting lowleakage current are, for example, BaTiO₃, SrTiO₃, KNO₃, LiNbO₃, Bi₄ Ti₃O₁₂, PbTiO₃, PbZrO₃, LaTiO₃, PbMgO₃, PbNbO₃, LaZrO₃, and the like.Preferably, dielectric film layer 121 comprises a polycrystalline layerof BaTiO₃ (BTO). BTO is preferred due to its low leakage currentcharacteristics and easily controllable microstructure.

Engaging dielectric film layer 121 is dielectric film layer 125.Dielectric film layer 125 is a ferroelectric material having a highdielectric constant, and is preferably a thin film layer of PbZr_(x)Ti_(1-x) O₃ (PZT) having a preferred thickness of about 200 nm. Toarrive at the embodiment depicted in FIG. 4b, PZT layer 125 is depositedupon dielectric film layer 121 by means of RF magnetron sputtering at asubstrate temperature of about 350° C. for about 30 minutes after whichPZT layer 125 is preferably furnace annealed at about 620° C. for about30 minutes.

Layer 121 serves as a "seed" to control nucleation of layer 125, thusgreatly increasing the dielectric constant of layer 125 and capacitiveelement 100.

Top amorphous layer 127 comprises an amorphous layer of a suitabledielectric material exhibiting low leakage current. Suitable dielectricmaterials which may be used in the invention are BaTiO₃, SrTiO₃, KNO₃,LiNbO₃, Bi₄ Ti₃ O₁₂, PbTiO₃, PbZrO₃, LaTiO₃, PbMgO₃, PbNbO₃, LaZrO₃, andthe like. A most preferred dielectric material for top amorphous layer127 is BaTiO₃ (BTO). Preferably, a thin film layer of BaTiO₃ (BTO)having a thickness of about 15 nm. is deposited upon PZT layer 125 bymeans of RF magnetron sputtering. Substrate temperatures from roomtemperature up to about 350° C. may be used. Layer 127 is amorphous andserves to limit current.

Upper metal electrode 130 may be formed upon top amorphous layer 127such that upper metal electrode 130 is in communication with topamorphous layer 127.

FIG. 4c is a block diagram and FIG. 5 is a cross-sectional view of apreferred embodiment of a high dielectric constant, low leakage, thinfilm capacitor 10 in accordance with the present invention. Substrate 15may be a semiconductor substrate, or may be constructed from adielectric, metal, or glass material. In constructing the embodimentshown in FIGS. 4c and 5, substrate 15 is a (100)-oriented p-Si wafer.Substrate 15 may include a oxide layer 17 which may, or may not, beoxidized in a wet O₂ atmosphere.

Platinum (Pt) layer 19 serves as a lower electrode for capacitor 10. Inthe embodiment illustrated in FIGS. 4c and 5, Pt layer 19 is a(111)-oriented film having a thickness of about 200 nm. According to themethod of the present invention, Pt layer 19 was deposited on oxidelayer 17 by means of RF magnetron sputtering carried out at a substratetemperature of about 350° C. However, other substrate temperaturesranging from about room temperature up to about 350° C. may be utilizedin the sputtering process. A pure Pt target of about one inch diameterwas sputtered in an Argon (Ar) ambient atmosphere having a base pressuregreater than about 2×10⁻⁶ Torr. The total Ar pressure and power duringPt deposition were kept at about 10 mTorr. and about 50 Wattsrespectively.

An initially amorphous (a) first thin film layer 21 comprising a layerof BaTiO₃ (BTO) having a thickness of about 15 nm. was deposited upon Ptlayer 19 by RF magnetron sputtering at a substrate temperature of about300° C. As will be readily apparent to those skilled in the art, othersubstrate temperatures ranging from about room temperature up to about350° C. may be employed in the sputtering process.

Polycrystalline second thin film layer 23 comprising a layer of BTOhaving a thickness of approximately 15 nm. was deposited upon first thinfilm layer 21 by means of RF magnetron sputtering carried out at asubstrate temperature of about 700° C. During the thermal treatment ofsecond thin film layer 23 described above, first thin film layer 21changed from an amorphous structure to a microcrystalline (m) structure.

PZT layer 25, having a thickness of about 200 nm. was deposited uponsecond thin film layer 23 by means of RF magnetron sputtering at asubstrate temperature of about 350° C. for about 30 minutes after whichPZT layer 25 was furnace annealed at about 620° C. for 30 minutes.

Top amorphous layer 27 comprising a thin film layer of BTO having athickness of about 15 nm. was deposited upon PZT layer 25 by means of RFmagnetron sputtering at a substrate temperature of about 350° C. As willbe readily apparent to those skilled in the art, other substratetemperatures ranging from about room temperature up to about 350° C. maybe employed in this process.

In practice, contact means 35 and 37, which are preferably gold (Au),may be disposed upon Pt layer 19 and upper electrode 30.

PREFERRED EMBODIMENT AND METHOD OF MANUFACTURE

PbZr_(x) Ti_(1-x) O₃ (PZT) is the preferred ferroelectric dielectricmaterial for use in forming layer 25 of the preferred embodiment 10 ofthe invention as depicted in FIG. 5. This embodiment, as well asalternate embodiments employing PZT as the ferroelectric dielectricmaterial, are suitable for applications including non-volatile memoriesand dynamic random access memories (DRAMs) due to the favorablehysteresis properties and high dielectric constant of PZT. Severalmethods of PZT thin film deposition may be employed in constructing thethin film PZT layer 25 of the present invention. These methods includechemical vapor deposition, chemical sol-gel process, laser ablation, andsputtering. Among these, sputtering is the preferred method because ofits simplicity and convenience. However, deviation in the composition offilms occurs from time to time due to the loss of Pb during thesputtering process. In addition, the interdiffusion between Pb and thebottom electrode, which deteriorates the structural and electricalproperties of the ferroelectric PZT capacitors, poses a problem inpractical application of the sputtering process.

The present invention addresses these, and other problems by providingan improved method for synthesis of PZT thin film layer 25 deposited onSi substrates using RF magnetron sputtering. According to the methods ofthe present invention, film composition, microstructure, dielectric, andelectrical properties are considered as a function of targetcomposition, deposition temperature and annealing conditions. The novelmulti-layer preferred structure 10 of the present inventionadvantageously combines PZT layer 25 with the nanolayer BaTiO₃ layers,21, 23, and 27, taking advantage of the superior electrical propertiesof the nanolayer BaTiO₃ structure. Further, the electrical properties ofPZT layer 25 are improved. Since both BaTiO₃ and PZT have the sameperovskite crystal structure, the degree of crystallinity of PZT filmlayer 25 for multi-layer structure 10 using BaTiO₃ as a seeding layer 23for PZT nucleation will be increased. Furthermore, the BaTiO₃ layers 23and 21 serve as barrier layers against the interdiffusion between Pb inthe PZT layer and lower electrode 19.

DETAILS OF METHOD

The substrate 15 employed in fabricating capacitor 10 of FIG. 5 was a(100)-oriented p-Si wafer with oxide layer 17 oxidized in a wet O₂atmosphere. The thickness of the SiO₂ layer comprising oxide layer 17was about 200 nm. After cleaning substrate 15 in trichloroethylene,acetone, methanol, and rinsing several times in 18 MΩ cm deionized (DI)water, followed by drying in a nitrogen jet, Pt layer 19 was depositedupon oxide layer 17 at substrate 15 by RF magnetron sputtering at 350°C. to form lower electrode 19 for ferroelectric PZT capacitor 10. Pt waschosen for lower electrode 19 because of its favorable properties as anelectrode material.

According to the method of the present invention, Pt layer 19 wasdeposited by the following method. A pure Pt target of one inch diameterwas sputtered in an Ar ambient with the base pressure better than 2×10⁻⁶Torr. The total Ar pressure and power during Pt deposition were kept at10 mTorr. and 50W, respectively. The thickness of the lower Pt electrode19 was around 200 nm. The resistivity of Pt measured by the four-pointprobe method was lower than 30 μΩ cm under such deposition conditionsand the Pt film was (111)-oriented.

                  TABLE 1                                                         ______________________________________                                        The sputtering conditions of PZT thin films                                   Parameters        Conditions                                                  ______________________________________                                        Target            PZT (one inch in diameter)                                  R.f. frequency    13.56 MHz                                                   Power density     100 W cm.sup.-2                                             Gas ratio         O.sub.2 /Ar + O.sub.2 = 1/16                                Total pressure    Ar + O.sub.2 = 8 mTorr.                                     Substrate temperature                                                                           25˜500° C.                                     Film thickness    100˜200 nm.                                           ______________________________________                                    

BaTiO₃ and ferroelectric PZT thin films were then deposited at differenttemperatures by using RF magnetron sputtering. Preparation of BaTiO₃thin films has been published previously in Q. X. Jia, L. H. Chang andW. A. Anderson, Thin Solid Films, 259 (1995) 264; Q. X. Jia, L. H. Changand W. A. Anderson, Ferroelectrics, 166 (1995) 111; and L. H. Chang andW. A. Anderson, 7th International Conference on Solid Films andSurfaces, Hsinchu, Taiwan, Dec. 12-16, 1994, all of which are herebyincorporated by reference.

To address the presented problem by the preparation of PZT films atelevated temperatures, i.e. substantial Pb loss, the present inventionprovides a method of synthesis of PZT thin films with different Pbcomposition from that of the PZT target.

According to the method of the present invention, a one inch diameterPZT target was prepared by mixing the raw oxide powders, PbO, ZrO₂, andTiO₂, inside a one inch diameter stainless steel cylinder and thenpressing at 38,000 pounds.

With a different atomic ratio of PbO, different PZT compositions, Pb₁.1(Zr₀.52 Ti₀.48)O₃, Pb₁.2 (Zr₀.52 Ti₀.48)O₃, and Pb₁.3 (Zr₀.52 Ti₀.48)O₃,were fabricated to arrive at the preferred embodiment of the method ofthe present invention. The starting PZT target with excess Pbcomposition was prepared specifically to compensate for the lead lossduring PZT deposition and/or post annealing processes.

The sputtering conditions of PZT thin films are listed in Table 1. Amongthose parameters listed in Table 1, target composition and substratetemperature were varied, while other parameters were fixed. Theas-deposited PZT films were subjected to post thermal treatment atdifferent temperatures from 500° C. to 700° C. with different times from15 min. to 90 min. This was done at one Atm. in a tube furnace in air.The thermal treatment was purposely conducted to obtain the perovskitephase of PZT films which gave a pyrochlore structure for theas-deposited processes.

The structural properties of the Pt, BaTiO₃ and PZT films were analyzedby X-ray diffraction (XRD) from a Nicolet/STOE diffractometer using CμKα radiation. The film composition was realized by energy-dispersiveX-ray analysis (EDAX). The leakage current density and hysteresisbehavior were determined by the current-voltage (I-V) andpolarization-electric field (P-E) measurements using a conventionalSawyer-Tower circuit operated at 60 Hz.

RESULTS

Single PZT Layer

By comparing films deposited using different PZT target compositions asdescribed above, the film composition, microstructure, and electricalproperties of the films were examined as a function of sputteringconditions and thermal treatment. Particular attention was paid to filmsdeposited on Pt bottom electrodes so that microstructure, dielectric,and electrical properties could all be measured on a single substrate,thereby more closely representing a typical memory device configuration,metal-ferroelectric-metal (MFM). Care was taken to maintainapproximately the same thickness among different samples to establish ameaningful comparison of compositional, structural, and electricalproperties.

FIG. 1 shows the annealing temperature dependence of the Pb contentratio, Pb/(Zr+Ti), with three different PZT target compositions,determined using EDAX. PZT films shown in FIG. 1 were prepared bysputtering at 350° C. with a power of 50W followed by furnace annealing.The dashed line indicates the composition of the desired stoichiometricPZT phase. The Ti/(Zr+Ti) ratio, which was not shown in FIG. 1, wasfound to be fairly close to the composition of the target material forvarious targets and a wide range of annealing temperatures from 500° C.to 700° C. However, the composition deviation of films from that of thetarget is great for the Pb/(Zr+Ti) ratio. Also, there is some scatterfor the Pb/(Zr+Ti) ratio through different thermal treatments. Ingeneral, about 20% loss of Pb content was observed after sputtering andpost-annealing treatments described above. The thermal evaporationprocess is believed to be the major factor resulting in thecompositional deviation.

For the PZT target with Pb/(Zr+Ti) of 1.1, there was a Pb deficiency ofabout 5% to 10% from the expected stoichiometric PZT perovskite phase.This resulted in a pyrochlore-type structure which gives poor dielectricand ferroelectric properties. On the contrary, excess Pb content of theprepared films from the PZT target with Pb/(Zr+Ti) ratio of 1.3, broughtabout lead oxide phases instead of the perovskite phase which caused asignificant increase of leakage current density. From these results, itwas determined that the PZT target with Pb/(Zr+Ti) ratio of 1.2 would bepreferred as the starting target to prepare the films with ferroelectricperovskite phase although about 2% to 3% of excess Pb is shown in FIG.1.

With Pb/(Zr+Ti) ratio of 1.2 for the PZT target, the substratetemperature was varied from 25° to 500° C. For the films deposited at asubstrate temperature of 500° C., significant Pb loss was detected,resulting in "leaky" films, and strange film morphology, which isbelieved to be due to the severe interdiffusion or interaction betweenthe deposited film and the underlying Pt layer, no matter what kind oftarget was used.

The XRD patterns for the PZT films deposited at different temperatureswithout any further thermal treatments is illustrated in FIG. 2. Thefilms deposited at 25° C. showed featureless XRD patterns. However, adiffraction peak at 2θ=29° was detected corresponding to the pyrochlore(222) phase for the as-deposited films at 350° C. This may be due to theformation of an oriented Pb-rich phase, caused by the excess Pbintroduced into the film. The resulting as-deposited films with asubstrate temperature of 350° C. generally had about 10% to 15% more Pbthan stoichiometric PZT.

Post-deposition annealing at a relatively higher temperature wasexpected to crystallize or transform the as-deposited amorphous films orpyrochlore phase into the perovskite phase. FIG. 3 shows the XRDpatterns for the PZT films deposited at different temperatures followedby different annealing temperatures. In FIG. 3a, the X-ray diffractionpeak shift from 2θ=28° to 2θ=31° was observed for the PZT filmsdeposited at 350° C. followed by furnace annealing at 620° C. for 30min. No significant peaks from other phases in the PZT films wereobserved. This corresponds to the transformation of the pyrochlore phaseto the perovskite phase. Preferred orientation of the perovskite phase,shown in FIG. 3a, was a mixture of (101) and (110).

In FIGS. 3b and 3c, the PZT films were deposited at 200° C. (Ts)followed by furnace annealing at 620° C. (Ta) (FIG. 3b) and 670° C. (Ta)(FIG. 3c) for 30 min., respectively, where crystallization was observedfor the films deposited at 200° C. At 670° C., crystalline peaks couldnot be unambiguously attributed to the perovskite phase because of theirbroadness and weak intensity, and the existence of small pyrochlorepeaks. For the furnace annealing at 700° C. for 30 min. of the filmsdeposited at 200° C., strange morphology with cracks and dots wasobserved. The crystallization of the film, while the amorphous films aresubjected to heat treatment as high as 700° C., may be accompanied byout-diffusion and evaporation of Pb, diffusion of film elements into thesubstrate, and diffusion of substrate material towards the film surface.The migration of Pb into the underlying layer and the movement of Pt upthrough PZT grain boundaries and cracks were observed in the film shownin FIG. 3d, where the thermal treatment at 670° C. for 30 min. wascarried out on the PZT films deposited at 25° C. A mixture of pyrochloreand perovskite phase was observed in FIG. 3d. Further, increasing theannealing temperature once again caused the severe interdiffusion of thePZT film with the substrate and substantial Pb loss. The low dielectricconstant for thermal treatment higher than 700° C. provides the evidencefor formation of an interface layer between the PZT film and Pt.

In summary, the PZT films deposited at 350° C. exhibited a high degreeof crystallinity as-deposited and required less annealing to produce thedesired perovskite phase, compared with those deposited at temperatureslower than 200° C. It was also observed that more Pb was retained infilms deposited at 350° C. (about 3% Pb rich) than in those deposited at250° C. (about 10% Pb deficient), after thermal treatment at 620° C. for30 min. This may be due in part to the low volatility of Pb in theperovskite phase, as opposed to in the amorphous phase, or incipientcrystallizing of the film. Thus, in comparison to films deposited at 25°C., those deposited at a temperature of 350° C. suffer less Pb loss andPt diffusion for thermal conditions necessary to obtain the desiredferroelectric perovskite phase, making a temperature of about 350° C.preferred for the method of the present invention.

PZT films gave leakage current (I_(L)) of 2×10⁻⁷ A/cm² @4×10⁵ V/cm,Table 2 illustrates the crystal structure types and Pb/(Zr+Ti) ratio forthe PZT films deposited on Pt/SiO₂ /Si with various processingconditions. From a stoichiometric viewpoint, the Pb/(Zr+Ti) ratio isless than 1.0 when the film forms a pyrochlore type structure. The XRDdata showed that the perovskite type structure could not be obtainedwhen the Pb content was low.

From these trials the preferred processing condition to obtain properstoichiometric PZT, desired ferroelectric perovskite phase, and a betterdielectric properties, is a PZT target having a Pb/(Zr+Ti) ratio of 1.2and depositing at 350° C., followed by a thermal treatment at 620° C.for 30 min. This conclusion is supported by data in Table 2.

                  TABLE 2                                                         ______________________________________                                        The crystal structure types and Pb/(Zr + Ti) ratio for                        the PZT films with various processing conditions                              Target            Annealing Film                                              Composition       Temperature                                                                             Composition                                                                           Microstructure                            Pb/(Zr + Ti)                                                                          Ts (° C.)                                                                        (° C.)                                                                           Pb/(Zr + Ti)                                                                          Type                                      ______________________________________                                        Pb(1.1) 25        620       0.92    pyrochlore                                Pb(1.1) 200       620       0.94    pyrochlore                                Pb(1.1) 350       620       0.95    pyrochlore +                                                                  Perovskite                                Pb(1.1) 25        670       0.90    pyrochlore                                Pb(1.1) 200       670       0.93    pyrochlore                                Pb(1.1) 350       670       0.94    pyrochlore +                                                                  Perovskite                                Pb(1.2) 25        620       0.98    Perovskite +                                                                  pyrochlore                                Pb(1.2) 200       620       1.02    Perovskite                                Pb(1.2) 350       620       1.02    Perovskite                                Pb(1.2) 25        650       0.97    pyrochlore +                                                                  Perovskite                                Pb(1.2) 200       650       1.02    Perovskite                                Pb(1.2) 350       650       1.02    Perovskite                                Pb(1.3) 25        620       1.08    Pyrochlore                                Pb(1.3) 200       620       1.08    Pyrochlore                                Pb(1.3) 350       620       1.10    PbO +                                                                         Perovskite                                Pb(1.3) 25        670       1.07    pyrochlore                                Pb(1.3) 200       670       1.08    pyrochlore                                Pb(1.3) 350       670       1.09    PbO +                                                                         Perovskite                                ______________________________________                                    

The ferroelectric properties of the PZT films on Pt/SiO₂ /Si substrateswere confirmed by the P-E hysteresis measurements. FIG. 4 shows atypical P-E hysteresis loop of the PZT film with optimized processingcondition. The remnant polarization is about 30 μC cm⁻² and coercivefield is about 80 kV cm⁻¹, corresponding to a coercive voltage of about0.8 V. A summary of the dielectric and electrical properties of the PZTfilms on a Pt/SiO₂ Si substrate with optimized processing conditions isgiven in Table 3. The lower than desired value of dielectric constant islikely due to the interdiffusion at the Pt interface and some loss of Pbat the surface, as will later be discussed.

                  TABLE 3                                                         ______________________________________                                        The dielectric and electrical properties of the                               PZT films with optimized processing condition                                 Leakage                          Remanent                                     current                                                                              Breakdown                 polari-                                                                              Coercive                              density                                                                              field     Dielectric                                                                             loss   zation Field                                 (A cm.sup.-2).sup.a                                                                  (V cm.sup.-1)                                                                           constant.sup.b                                                                         tan δ                                                                          (μC cm.sup.-2)                                                                    (kV/cm.sup.2)                         ______________________________________                                        2 × 10.sup.-7                                                                  1 × 10.sup.6                                                                      548      0.03   30     80                                    ______________________________________                                         .sup.a At a field of 4 × 10.sup.5 V cm.sup.-1.                          .sup.b At 1.0 MHz.                                                       

Multi-layer Structure

Interaction between PZT and the bottom electrode deteriorates theelectrical properties and also reduces the crystallinity of the PZTfilms. In order to overcome this problem, the preferred multi-layerstructure combined PZT and BaTiO₃ layers to suppress the interdiffusionbetween Pt and Pb and also to increase the crystallinity of the PZTlayer. The cross-section of the multi-layer capacitor structure is shownin FIG. 5. The bottom BatiO₃ layer deposited at 300° C. was amorphous(a); however, it became microcrystalline (m) owing to the thermaltreatment during later polycrystalline (p) BaTiO₃ deposition at 700° C.The PZT layer was then deposited at 350° C. followed by furnaceannealing at 620° C. for 30 min. The sample was finished by RFsputtering of the top amorphous BaTiO₃ layer with the substratetemperature at 350° C. Typical electrical properties are given in Table4.

                  TABLE 4                                                         ______________________________________                                        Electrical properties of the multi-layer structure                                   Capacitance                                                            Thickness                                                                            density    Dielectric                                                                              Leakage current density                           (nm)   (× 10.sup.5 pF cm.sup.-2)                                                          constant.sup.a                                                                          (A cm.sup.-2) at 1.5 × 10.sup.5 V                                       cm.sup.-1                                         ______________________________________                                        245    16         442       2 × 10.sup.-9                               ______________________________________                                         .sup.a At 1.0 MHz.                                                       

The electrical properties of the nanolayer BaTiO₃ structure, a-BaTiO₃/poly-BaTiO₃ /micro-BaTiO₃ are superior. By taking advantage of the bestproperties from both amorphous and polycrystalline BaTiO₃ structures,the nanolayer a-BaTiO₃ /poly-BaTiO₃ /micro-BaTiO₃, (thickness 20 nm/280nm/20 nm), with a very low leakage current of 2×10⁻⁶ A cm⁻² at a fieldof 4×10⁵ V cm⁻¹ and an effective dielectric constant of 259 wasachieved.

The relative dielectric constant for the BaTiO₃ film deposited at atemperature of 200° C. was 21.6 but about 220 after thermal treatment at700° C. The second BaTiO₃ layer deposited at 700° C. to form apolycrystalline structure had a dielectric constant of about 400. Forthe single BaTiO₃ layer deposited at 700° C., the dielectric constant ofthe BaTiO₃ film did not give values higher than 300 owing to theinteraction between the BaTiO₃ film and the bottom electrode, whichformed some complex compounds with very low values of dielectricconstant at the interface. The dielectric constant of the PZT layer,however, was assumed to be 548, the same as that of the single PZT film.The termination layer of the capacitors, deposited at 350° C. withdielectric constant of about 40, was amorphous since there was nofurther high temperature treatment. The film thickness of each layer wasabout 15 nm., 15 nm., 200 nm., and 15 nm. corresponding to themicrocrystal BaTiO₃, polycrystalline BaTiO₃, PZT, and top amorphousBaTiO₃ layers, respectively.

The effective capacitance of the multi-layer structure is a seriesconnection of micro-BaTiO₃, poly-BaTiO₃, PZT and a-BaTiO₃, dielectrics.Theoretically, the effective dielectric constant ε_(eff), can beexpressed as ##EQU1## where t_(m), t_(p), t_(z), and t_(a) are thethicknesses of micro-BaTiO3, poly-BaTiO₃, PZT and a-BaTiO₃ layers,respectively. The ε_(m), ε_(p), ε_(z), and ε_(a), denote the dielectricconstants for micro-BaTiO₃, poly-BaTiO₃, PZT and a-BaTiO₃ layers,respectively. The theoretical evaluation of ε_(eff) was calculated basedon the electrical measurement on the multi-layer capacitors to be 400which gave the ε_(z) to be about 880 if the relative dielectricconstants of BaTiO₃ layers are fixed.

The large increase in the relative dielectric constant of the PZT layermay be due to the increase of the crystallinity of the PZT layer withthis multi-layer structure. A higher degree of crystallinity wasobtained for the PZT film with a multi-layer structure due to the use ofthe seeding poly-BaTiO₃ layer.

The interdiffusion problem was estimated by conducting AES profilingmeasurements of single and multi-layer PZT on a Pt bottom electrode. Thediffusion of Pb into the Pt bottom electrode was observed for the singlePZT layer. However, for the multi-layer structure, the BaTiO₃ layersprevented interdiffusion between Pb and Pt.

One of the most significant advantages of the multi-layer structure ofthe present invention is the significant reduction of the leakagecurrent density compared to the single PZT layer structure. The leakagecurrent density in the range of 10⁻⁸ A cm⁻² at a field intensity 1×10⁵ Vcm⁻¹ was obtained for the multi-layer structure, which is about twoorders of magnitude lower compared with the single PZT layer discussedin the previous section. FIG. 6 shows the I-V characteristics of thesingle PZT and multi-layer structures.

The significant reduction in the leakage current density whileaccomplishing such a high effective dielectric constant by using themulti-layer structure could be attributed to the following.

The first micro-crystalline layer of BTO serves as a protective bufferbetween the Pt electrode and subsequently deposited thin film layers. Atthe same time the first microcrystalline layer of BTO providesadvantageous chemical and crystal structure upon which further layersmay be deposited.

In addition, during polycrystalline BaTiO₃ film deposition, charge trapsgenerated during the amorphous BaTiO₃ film deposition may be annealedout. This results in lowering of the leakage current density and anincrease in the relative dielectric constant of the bottom BaTiO₃ layer.Furthermore, the polycrystalline BaTiO₃ layer may serve as the seed forthe nucleation of PZT due to the same perovskite crystal structure.Finally, a top thin amorphous BaTiO₃ layer was deposited on the PZT toachieve the termination of the grain boundaries in the polycrystallinefilms which usually caused much higher leakage current.

CONCLUSION

The influence of the processing parameters on the PZT film propertieswas in terms of sputtering target composition, deposition temperature,and annealing conditions is addressed by the teachings of the presentinvention. The Pb content impacted the crystallization in such a waythat the presence of excess Pb gave higher ease of perovskite phaseformation. Therefore, under the conditions used, and described herein,in the fabrication of one embodiment of the invention, the PZT targetwith Pb/(Zr+Ti) ratio of 1.2 provided the preferred starting target toobtain a PZT film with a ferroelectric perovskite phase. The idealPb/(Zr+Ti) ratio is dependent on the processing parameters employed in agiven deposition process. Those skilled in the art will recognize thatthe teachings of the present invention with regard to target preparationto reduce Pb loss allow for different preferred Pb/(Zr+Ti) ratios whenother fabrication conditions are employed.

The single layer PZT obtained under the optimized processing parametershowed ferroelectric behavior with a remanent polarization of 30 Cμ cm⁻²on a coercive field of 80 kV cm⁻¹. The dielectric constant of about 548was obtained for this film with an Au/PZT/SiO₂ /Si structure. Theleakage current at a field of 4×10⁵ V cm⁻¹ was about 2×10⁻⁷ A cm⁻².

The structural and electrical properties of the PZT layer were furtherimproved by employing the novel multi-layer structure which combined thePZT with the nanolayer BaTiO₃ layers. With this multi-layer structure,the leakage current density was reduced by two orders of magnitude andthe effective dielectric constant of the entire structure was estimatedto be around 442 which gave a relative dielectric constant of the PZTlayer to be about 880. The increase of the crystallinity of the PZTlayer and suppression of the interdiffusion between Pb and Pt were twomajor contributors to the superior electrical properties of themulti-layer structure.

It is intended that the above description of preferred embodiments ofthe structure and method of the present invention are but one or twoenabling best mode embodiments for implementing the invention. Othermodifications and variations are likely to be conceived of by thoseskilled in the art upon a reading of the preferred embodiments and aconsideration of the appended drawings. These modifications andvariations still fall within the breadth and scope of the disclosure ofthe present invention.

We claim:
 1. A capacitor comprising a multilayer structure between twoelectrodes, the multilayer structure comprising:a first layer and asecond layer of a first dielectric material having a first dielectricconstant; the first layer having an internal structure affectingformation of a crystalline structure of the second layer; thecrystalline structure of the second layer affecting formation of aperovskite crystalline structure of a ferroelectric layer deposited onthe second layer; the ferroelectric layer having a second dielectricconstant higher than the first dielectric constant; and an amorphouslayer serving to limit leakage current in the capacitor by terminatinggrain boundaries in the ferroelectric layer.
 2. The multilayer structureof claim 1, wherein the internal structure of the first layer ismicrocrystalline.
 3. The multilayer structure of claim 1, wherein thecrystalline structure of the second layer is polycrystalline.
 4. Themultilayer structure of claim 1, wherein the ferroelectric layer is madeof PbZr_(x) Ti_(1-x) O₃, wherein x represents a molar fraction ofzirconium.
 5. The multilayer structure of claim 1, wherein the firstdielectric material is selected from the group consisting of BaTiO₃,SrTiO₃, KNO₃, LiNbO₃, Bi₄ Ti₃ O₁₂, PbTiO₃, PbZrO₃, LaTiO₃, PbMgO₃,PbNbO₃ and LaZrO₃.
 6. The multilayer structure of claim 1 furthercomprising a first electrode contacting the first layer and a secondelectrode contacting the amorphous layer.
 7. The multilayer structure ofclaim 6, wherein the first and the second electrodes are made of amaterial selected from the group consisting of Pt, Pd, Au, Ag, Al, Cu,Pb, Ru and metallic oxides.
 8. A multilayer structure disposed between afirst electrode and a second electrode, the multilayer structurecomprising:a microcrystalline layer of a first dielectric materialdeposited on the first electrode; a polycrystalline layer of a firstdielectric material deposited on the microcrystalline layer; a layer ofa second dielectric material deposited on the polycrystalline layer, thesecond dielectric material having a dielectric constant higher than thatof the first dielectric material and having a crystalline structuresubstantially corresponding to a perovskite phase; and an amorphouslayer of the first dielectric material deposited between the secondelectrode and the layer of the second dielectric material.
 9. Themultilayer structure of claim 8, wherein the first dielectric materialis selected from the group consisting of BaTiO₃, SrTiO₃, KNO₃, LiNbO₃,Bi₄ Ti₃ O₁₂, PbTiO₃, PbZrO₃, LaTiO₃, PbMgO₃, PbNbO₃ and LaZrO₃.
 10. Themultilayer structure of claim 8, wherein the second material is PbZr_(x)Ti_(1-x) O₃, wherein x represents a molar fraction of zirconium.
 11. Themultilayer structure of claim 8, wherein the thickness of the structureis less than one micron.
 12. The multilayer structure of claim 8,wherein the first and the second electrodes are made of a materialselected from the group consisting of Pt, Pd, Au, Ag, Al, Cu, Pb, Ru andmetallic oxides.